1. Field of the Invention
This invention relates to optical wavelength-converting devices using nonlinear optical materials, and more particularly to an optical waveguide device that allows guided-wave second-harmonic generation in Cerenkov radiation mode.
2. Description of the Related Art
Recently, many efforts have been devoted to researching and developing optical elements that generate the second-harmonic wave, by using nonlinear optical crystal, to obtain a short-wavelength light source. To make the light source smaller and less power-consuming, continual attempts have been made to use semiconductor laser in generating the fundamental wave and to form an optical wavelength in the optical crystal wave. The nonlinear optical crystal used is arranged so as to define a stripe-shaped optical waveguide on the top surface of the substrate.
A typical optical wavelength-converting device of this type is disclosed in the article "Second harmonic generation using proton-exchanged LiNbO.sub.3 waveguide", by T. Taniuchi et al., Optoelectronics, 1987, Vol. 2, No. 1, p.53-58. With the arrangement shown in this literature, semiconductor laser light is first produced as the fundamental wave, which is then converted into a second-harmonic wave (e.g., blue light) radiated within the substrate as a Cerenkov radiation wave. The second-harmonic wave radiates diagonally downward with respect to the propagation direction of the fundamental wave, which is propagated in the guiding direction of the waveguide formed on the substrate.
Such conventional Cerenkov radiation-mode optical wavelength-converting devices, however, have poor conversion efficiency. The reason for this is considered as follows: while the phase-matching condition between the fundamental wave and the second-harmonic wave can be completely satisfied along the propagation direction of the fundamental wave in the second-harmonic wave Cerenkov radiation, the phase-matching condition cannot always be satisfied along the vertical direction perpendicular to the substrate's surface. Such phase-mismatching reduces the radiation efficiency of the second harmonic wave considerably.
The generation of such crystalline phase mismatching is closely related to the physical layer structure of a waveguide, including the material, depth, and width. In general, the physical layer structure of the waveguide in a Cerenkov-type optical wavelength-converting device is designed mainly to fulfill conditions for the generation of the second-harmonic wave, the conditions being known as the Cerenkov conditions. It is not allowed to modify the waveguide parameter in such a way that its design does not meet the Cerenkov conditions. That is, the waveguide cannot be designed freely to prevent the phase mismatching; improvements in the waveguide structure for no mismatching has to be strictly limited in terms of flexibility. This means that a complete removal of the phase-mismatching is intrinsically difficult. With this backdrop, attempts to increase the optical conversion efficiency to the desired level has been successful only in extremely limited range.